1. Technical Field
This disclosure is directed toward a method for providing mercury sorption during gypsum calcination, wherein the method utilizes a mercury sorbent such as activated carbon and/or derivatives thereof to capture mercury released from the gypsum during calcination.
2. Description of the Related Art
Emissions from electricity-generating facilities such as sulfur dioxide (SO2), nitrogen oxide (NOx), and mercury (Hg) are known to cause detrimental impacts to human health and the environment. Coal-burning power plants currently account for the majority of SO2 emissions in the United States. To reduce SO2 emissions, Congress passed the Clean Air Act Amendment of 1990 (the “Act”) to regulate SO2 emissions generated by power plants.
In order to comply with the Act, power plants generally employ one of two strategies to control SO2 emissions: 1) using high-cost, low-sulfur coal; or 2) installing a flue gas desulfurization (FGD) system to absorb the SO2 in the flue gas stream before it is released to the atmosphere. Currently, about 22% of coal-burning power plants in the U.S. use FGD systems to achieve the mandated SO2 emission level, and this percentage is expected to double in the next seven years in response to future regulations.
The most widely used FGD systems are wet scrubbers using calcium-based sorbent, such as lime or limestone, together with forced oxidation. As the SO2 laden flue gas passes through the FGD system, SO2 is absorbed when exposed to water and limestone as various combinations of calcium sulfate dihydrate (CaSO4.2H2O), bassanite (2CaSO4.H2O), and anhydrite (CaSO4). Typically, the precipitate also includes calcite (CaCO3), and minor amounts of fly ash and carbon grains. The precipitate containing hydrated forms of calcium sulfate is referred to as FGD gypsum, or synthetic gypsum, in order to be differentiated from gypsum that occurs as a natural mineral, which consists essentially of calcium sulfate dihydrate. Commercial applications of FGD gypsum include, but are not limited to, wallboard, structural fill, aggregate, mining applications, Portland cement, plaster, glass making, pharmaceutical filler, paper, plastic, floor systems, fuel additive, and agricultural applications such as soil stabilization and soil neutralization.
One of the most popular applications of FGD gypsum is wallboard manufacturing, wherein raw FGD gypsum is calcined (or calcinated) at 250 to 380° F. in a reactor to form stucco, which is further rehydrated and processed into wallboard. In fact, approximately 70% of raw FGD gypsum is used in commercial wallboard manufacturing.
In addition to SO2, trace levels of mercury may also be present in the flue gas being treated by the wet FGD system. As gypsum precipitates in the FGD system, a portion of the mercury may be captured with the FGD gypsum.
The mercury found in FGD gypsum generally originates from coal. The U.S. Geological Survey (USGS) has assembled a nationwide coal information database containing more than 7000 coal samples. Statistics of the analysis of those samples indicates an average mercury content of 170 ppb, with the maximum mercury content being as high as 1.8 ppm.
Mercury in coal is released to a flue gas stream when the coal is combusted. The resulting “fugitive” mercury, either in the form of elemental mercury or ionized mercury, can be captured using existing control methods. The most popular method comprises injecting a mercury sorbent, such as activated carbon, into the flue gas stream to sorb the fugitive mercury, followed by subsequent capture of the mercury-containing activated carbon by particulate control equipment.
Despite the ability of activated carbon to sorb fugitive mercury, activated carbon injection generally requires the use of large quantities of activated carbon, which is costly, and may not sufficiently remove all of the fugitive mercury from the flue gas stream. Therefore, FGD gypsum generally contains a certain amount of mercury which may be released to the environment during the calcination process.
The amount of mercury released during the calcination of FGD gypsum varies. Studies indicate that from 0.6% to 50% of the mercury contained in the FGD gypsum is released to the environment during calcination, depending on the type and initial mercury content of the FGD gypsum.
While activated carbon injection is the most mature technology in controlling mercury emissions from coal combustion, its application to control the mercury release during gypsum calcination is not yet available. As discussed above, a large quantity of activated carbon is required for capturing fugitive mercury in a flue gas stream. However, such a large quantity of activated carbon, when applied to the calcination of a gypsum, would presumably affect the quality of the stucco produced, including for example, the appearance, cube strength, rehydration time, and foam generation/degeneration characteristics of the stucco.
Therefore, there is a need for a method that provides effective mercury release control during the manufacture of wallboard from FGD gypsum. More generally, there is a need for a method that provides effective mercury release control during gypsum calcination. Further, there is a need to provide effective mercury release control during gypsum calcination without substantially affecting the quality of the resulting stucco.